Poly(butylene terephthalate), or PBT, is an important polyester fiber; it is also an important crystalline thermoplastic resin. Polyester fibers, which show good resistance to most mineral acids and display excellent resistance to cleaning solvents and surfactants, are now the world's leading synthetic fibers. Crystalline thermoplastic poly(butylene terephthalate), along with other crystalline thermoplastic poly(alkylene terephthalates), such as poly(ethylene terephthalates), exhibits many favorable physical, mechanical, and electrical properties, and is considered one of the most important engineering plastics.
Poly(butylene terephthalate) is manufactured commercially by the so-called DMT (dimethyl terephthalate) or TPA (terephthalic acid) process, both of which typically use tetrabutyl titanate as the catalyst.
In the conventional DMT process for making poly(butylene terephthalate), dimethyl terephthalate (DMT) and butylene glycol (i.e., 1,4-butanediol, BDO) are used as the raw materials which are reacted at 220.degree..about.260.degree. C., using tetrabutyl titanate as the catalyst to effectuate a transesterification reaction and form an intermediate product, which is bis(2-hydroxybutyl terephthalate) (BHBT). The intermediate product bis(2-hydroxybutyl terephthalate) is then subject to a high temperature (250.degree..about.290.degree. C.) and high vacuum (less than 1 torr) to effectuate a condensation polymerization. After the reaction, the unreacted butylene glycol is removed to obtain the poly(butylene terephthalate) final product.
The TPA process has not been as commercially successful as the DMT process for the manufacturing of poly(butylene terephthalate). In the conventional TPA process, terephthalic acid (TPA) and butylene glycol are used as the raw material which are reacted at 220.degree..about.265.degree. C. to effectuate a transesterification reaction, using tetrabutyl titanate as the catalyst. An intermediate product of bis(2-hydroxybutyl terephthalate) is formed after dehydration of the terephthalic acid (TPA) and butylene glycol as a result of the esterification reaction therebetween. The intermediate product of bis(2-hydroxybutyl terephthalate) is similarly subject to a high temperature (250.degree..about.290.degree. C.) and high vacuum (less than 1 torn) condensation polymerization. After the completion of the reaction, the unreacted butylene glycol is removed from the reaction product to obtain the poly(butylene terephthalate) final product.
Because of the high commercial value and large volume of poly(butylene terephthalate), any incremental improvement in the production rate thereof is very significant. Therefore, it is important to improve the performance of the tetrabutyl titanate based catalyst composition. Furthermore, an increased production rate would also reduce the time during which the expensive tetrabutyl titanate catalyst is subject to the possibility of being poisoned, per unit volume of poly(butylene terephthalate) produced. This can also result in important cost savings in the production of poly(butylene terephthalate).